FIG. 1 shows a traditional broadcast CATV (cable television) system, generally designated by the reference numeral 10. CATV system 10 includes a headend 12 which is responsible for broadcasting analog video to all subscribers connected to the system. A headend might support from a few hundred homes in a rural area to hundreds of thousands of homes in a metropolitan area. Headend 12 is connected to multiple neighborhood nodes 14 by trunk lines 16. Traditional trunk lines include microwave links and coaxial cables, often associated with repeaters 17. Each neighborhood node serves the homes of a limited neighborhood area. In many traditional systems, however, neighborhood nodes might each serve several thousand homes.
From the neighborhood nodes, connections to homes are made through coaxial plants 18. A coaxial plant comprises multiple active coaxial feeders 19, each tapped by multiple passive coaxial drop cables that reach individual subscribers.
CATV system 10 is a one-way delivery system based on passband transmission. In the United States, each passband consists of a downstream 6 MHz channel, typically between 50 and 450 MHz. The 5 MHz through 42 MHz spectrum is reserved for upstream traffic. Many systems do not utilize the upstream frequencies.
In recent years, fiber-optic cable has been deployed very aggressively to replace traditional trunks between headends and neighborhood nodes. Optoelectronic conversion equipment is provided at the neighborhood node to make use of the coaxial cable plant extending to the individual homes. Fiber cabling increases reliability, reduces noise problems, and decreases maintenance costs. This type of distribution system is referred to as having a "hybrid fiber/coax" (HFC) architecture.
In conjunction with the conversion to fiber trunks, the number of homes supported by each neighborhood node has dropped to between 500 and 2000 homes. This is much smaller than the number of homes supported as little as 10 years ago.
Public HFC networks are being deployed not only by CATV companies, but also by telephone companies. The HFC networks deployed by telephone companies and some CATV operators are switched systems designed to support both broadcast video and switched broadband digital communication services. Switched systems such as this typically support much fewer homes per neighborhood node than traditional systems. They also provide additional downstream channels for interactive services. Such additional channels are typically in the 450 MHz to 750 MHz range, and could extend up to 1 GHz in the future. While analog broadcast channels will initially occupy the spectrum from 50 MHz to 450 MHz, reallocations will probably be made as interactive channels and services become available.
Newer, switched systems provide two-way communications using a back-channel which typically operates in the range of 5 MHz to 42 MHz. In addition, switched systems are capable of providing independent information services to individual subscribers. For instance, each home can choose its own information stream, such as a selected video or motion picture, independent of other homes and independent of broadcast schedules.
Some switched public networks use asynchronous transfer mode (ATM) cell transmission. This technology uses data cells with a fixed length of 53 bytes to reduce network latency and allow better statistical multiplexing of information on a given medium than available when using larger packets of variable length. The fixed length of the cells also simplifies switching them into and out of data media operating at different data rates.
FIG. 2 shows an ATM cell 21. The cell contains a 5-byte header 22 and a 48-byte information field or payload 24. For switching or routing purposes, only the header is significant. The first four bits of the first byte of the header contain a generic flow control field, designated GFC, which is currently not defined. It is intended to control the flow of traffic in a shared media network. The next 24 bits (the last half of byte one, bytes two and three, and the first half of byte four) make up the ATM virtual channel number, also referred to herein as a numeric routing indicator or a VPI/VCI value. The numeric routing indicator is made up two subfields: a virtual path identifier VPI and a virtual channel identifier VCI. The VPI is formed by the first byte of the numeric routing indicator. The VCI is formed by the second and third bytes. The next three bits, designated PT for payload type, indicate the type of information carried by the cell. The last bit of the header's byte four, CLP, indicates the cell loss priority as set by a user or by the network. This bit indicates the eligibility of the cell for discard by the network under congested conditions. The last byte of the header, HEC, is the header error control field. This is an error-correcting code calculated across the previous four bytes of the header. The HEC does not provide error checking or correction for the payload. If such checking or correction is desired, it must be performed at a higher protocol layer.
ATM networking depends on the establishment of virtual connections. An ATM virtual connection is a series of links between physical devices in a network. ATM uses virtual channels (VCs) and virtual paths (VPs) for routing cells through such physical devices. A virtual channel is a connection between two communicating ATM entities. It may consist of a concatenation of several ATM links. All communications between two end points proceed along a one or more virtual channels. Each virtual channel preserves cell sequence and is guaranteed to provide a specified data rate. A virtual path is a group of virtual channels. Virtual paths provide a convenient way of bundling traffic all heading in the same destination. Certain types of switching equipment (referred to as VP cross-connect switches) only need to check the VPI portion of a cell header to route the cell rather than the entire three-byte address. For instance, a single VPI might be used to indicate a path between two related offices. The VCIs would be used only within the offices to determine destinations within the offices.
In traveling from one end-point to another, a cell usually passes through one or more ATM node switches. A switch has a plurality of physical communication ports for communication with other switches or with individual end point devices. Each port, however, is connected to only a single ATM device. When a switch receives a cell, it routes the cell to the appropriate port simply by checking its VPI/VCI value, which has meaning only to the switch itself. By looking at the VPI and VCI of a cell, the switch can determine to which port the cell should be routed. Before actually sending the cell, the switch replaces the VPI/VCI value of the cell with that which will be needed at the next switch.
In order for this switching to occur, a virtual connection must have already been established through all of the involved switches. This occurs using "call setup" messages. To set up a virtual connection, an originating end point device exchanges "signaling" messages with a destination end point device and with the intervening switches. These messages contain detailed information about the end point devices and the intervening switches, including their ATM addresses. All devices along the desired path store this information and associate it with a specific virtual connection, specified within each device by a particular VPI/VCI combination. Subsequent data communications can then take place without any further specification of the detailed addressing information, with only the VPIs and VCIs.
The description given above is merely an overview. Further, more detailed information can be found in Asynchronous Transfer Mode: Bandwidth for the Future, by Jim Lane (Telco Systems, 1992); and in Asynchronous Transfer Mode: Solution for Broadband ISDN, 2d ed., by Martin de Prycker (Ellis Norwood, 1993). Both of these references are incorporated by reference.
The physical network architecture shown in FIG. 1 creates some practical difficulties in implementing an ATM protocol. One such difficulty is that a typical ATM architecture requires a switch port for every physical end point device. This can be expensive in a public system in which every home in a very large metropolitan area might have an end point device. Another potential difficulty arises from the fact that upstream and downstream communications in the system of FIG. 1 take place over different channels. A typical ATM architecture, however, expects that a node switch will be able to conduct both upstream and downstream communications with a device over the same switch port. These difficulties are solved by the architecture and methodical steps presented below.